Fix KeyError in _HandleCoaPacket for 0.0.0.0 host

[mmakaay]

trafficstars

When defining a client host 0.0.0.0 for a CoA radius Server, then a KeyError will occur when handling a CoA packet.

File ".../pyrad/server.py", line 250, in _HandleCoaPacket
    pkt.secret = self.hosts[pkt.source[0]].secret
                 ~~~~~~~~~~^^^^^^^^^^^^^^^
KeyError: '172.16.0.100'

The reason seems to be that _AddSecret() was added at some point to support using 0.0.0.0 as a host, but the pkt.secret = ... assignment that it ought to replace was not removed.

This patch fixes the behavior by removing the stale assignment.